The invention relates to aneroid bellows assemblies of the type used in emergency oxygen systems in aircraft. In commercial aircraft, it is required that crew members wear oxygen masks at appropriate times and that oxygen be supplied in increasing amounts if the cabin pressure is reduced below the normally maintained pressure of 11.8 psia associated with an altitude of 6,000 feet. It is also required that the rate of flow of oxygen increase substantially at pressures of less than about 7.24 psia, which corresponds to an altitude of 18,000 feet.
Aneroid assemblies capable of controlling the flow of oxygen in the above manner have been sold for many years by Scott Aviation Corp. of Lancaster, New York. Typically, such devices have included an evacuated bellows spring mounted internally of an open ended tubular housing and affixed at one of its ends to the closed end of the housing. The bellows expands axially as ambient pressure decreases. An end plate attached to the free end of the bellows spring overlies the open end rim of the housing and extends radially outwardly of the housing where it is engaged by a coil spring which encircles the housing. It is important that the coil spring the deflected a precise distance in order to insure that the movable tip end of the bellows assembly will move as necessary to control the flow of oxygen in the manner desired. Usual manufacturing tolerances in the coil spring, housing and bellows portions of the assembly are much greater than those needed to assure proper functioning of the various parts after assembly and thus, it has been necessary to sort the parts by size and selectively fit them to each other or, alternatively, to remove material from one of them. Such operations are very time consuming and expensive.